Like nuclear reactor installations are well-known and comprise both installations of the so-called "loop" type and installations of the so-called "pool" type. As cooling fluid for dissipating heat generated in the reactor core for example, liquid sodium may be used as, for example, in a nuclear reactor of the fast breeder type. The liquid sodium transfers the heat from the primary circuit in the heat exchangers between the loops of the primary circuit and the respective loops of the secondary circuit heat to the fluid in the secondary circuit, which fluid is, for example, also liquid sodium. The heat thus transferred to the secondary circuit is subsequently used, for example, for the conversion of water to steam in one or more steam generators coupled to the secondary circuit for heat transfer.
When the nuclear reactor is suddenly stopped or cut off, the heat production in the core quickly diminishes until it is only a few percent of the nominal power. This last value decreases only gradually in a period which may vary from a few hours to some days. The heat which is still generated in the reactor when it is cut off is called decay heat. Although the decay heat is only a few percent (for example, 1-3%) of the design power of the reactor, the decay heat may be considerable, particularly in recently proposed reactors having a very great design power. If the decay heat is not dissipated from the core, the reactor core will become superheated, with highly undesirable consequences: the stainless steel pins wherein the nuclear fuel is present in the reactor core could, for example, melt locally, resulting in reactions of the nuclear fuel with the sodium of the cooling fluid and the entire primary cooling fluid circuit could be strongly contaminated.
For the dissipation of the decay heat the normal cooling system of the reactor which usually is also employed for the dissipation of the heat generated during operation could serve. Since this manner of dissipation of the decay heat alone is not considered safe enough, extra provisions can be made. In a fast breeder type reactor as now under construction, for example, the decay heat is transferred via the primary circuit to the secondary circuit and thence to the water-steam system. When the reactor is cut off, the pumps of this entire system continue to work slowly for this purpose. As an addition to this system there are also provided immersion coolers which are switched on when the reactor is cut off.
A drawback of the utilization of the entire system of primary circuit, secondary circuit and water-steam system for the dissipation of the decay heat is that the secondary circuit and the water-steam system constitute part of the protective system of the nuclear reactor, as a result of which, beside the usually already set reliability requirements, extremely high safety requirements are set by the authorities. As a result of this, a scaling-up for future nuclear reactors is very difficult from a technical point of view.
Now, the object of invention is to provide a nuclear reactor installation wherein an alternative provision is made for the dissipation of the decay heat, so that the entire system of primary circuit, secondary circuit and water-steam system is not needed for the decay heat dissipation, while nevertheless there is ensured an installation satisfying the highest safety and reliability requirements.